Encoding apparatus in which flat subblocks are identified in blocks of a motion signal prior to coding the blocks and complementary decoding apparatus

ABSTRACT

In a picture signal encoding and/or decoding apparatus, respective blocks composing of 8×8 pixels are segmented into four subblocks composing of 4×4 pixels. A flatness of picture data is judged for every subblock. New picture data is generated by folding flat subblocks indicated with logic 1 and unflat subblocks indicated with logic 0, and then this picture data obtained by folding is discrete cosine transformed. Whereas it is able to high efficient encoding.

This is a divisional of application Ser. No. 07/942,927, filed Sep. 10,1992, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to an apparatus for compressing a picture signal,and an apparatus for expanding a compressed picture signal, and, moreparticularly, to an apparatus that is suitable for use in applicationsin which the picture signal being compressed is to be recorded, and inwhich the compressed picture signal being expanded has been reproduced.

It is known to compress a picture signal by dividing the picture signalinto blocks of 8×8 pixels (=8 pixels×8 lines), and subjecting the blocksto processing by means of a discrete cosine transform (DCT), quantizing,and variable length coding the resulting transform coefficients, whichare then recorded on a recording medium, e.g. a disc. The compressedpicture signal recorded on the disc is then reproduced from the disc,variable length decoded, inverse-quantized, and inverse-discrete cosinetransformed to reconstruct the original picture signal.

It is desirable to provide a recording medium that has a short accesstime and a large capacity because a motion picture, for example,requires that a large quantity of information to be stored. Presently,an NTSC video signal, for example, can be recorded on and reproducedfrom a conventional video disc. When it is desired to record the digitalmotion picture signal on a smaller disc than a conventional video disc,the motion picture signal must be subject to high efficiencycompression, and the reproduced motion pictures signal must be capableof being expanded efficiently.

To answer this problem, there have been proposed some methods forcompressing the motion picture signal to be recorded with highefficiency. One of these methods is that proposed by the moving pictureexperts group (MPEG). The MPEG method detects a motion vector for eachblock of the motion picture signal, and generates a prediction block byapplying motion compensation to a prediction picture according to themotion vector. This reduces redundancy in the motion picture signal inthe time domain. In addition, the block of prediction errors betweeneach block of the present picture and its corresponding prediction blockis subject to a discrete cosine transform, and the resulting transformcoefficients are quantized, to reduce redundancy in the motion picturesignal in the spatial domain.

Attempting to increase the compression efficiency by enlarging thequantizing step size by which the transform coefficients are quantizedresults in larger quantizing errors. Larger quantizing errors make noisethe in flat portions of the picture (i.e. the portions of the picture inwhich there is little detail) more obvious.

Further, in a conventional apparatus for compressing a motion picturesignal, the differential vector between the motion vectors of the targetblock and the left side block thereof is encoded on encoding the motionvector of prescribed each block. Therefore, when there are many targetsto be imaged in a picture, the motions of which are different eachother, the quantity of prediction error information between the currentpicture and the prediction picture is increased, which degrades thecompression efficiency.

Furthermore, in this situation, there is a problem that different pansof the block can have motions that are different from each other, whichdegrades the prediction accuracy.

To remedy this problem, it has been suggested that each of the 8×8blocks be divided into four 4×4 subblocks, that a motion vector bedetermined for each subblock, and that the motion of the block becompensated by using the resulting four motion vectors. However thisproposal degrades the compression efficiency because of the increasednumber of motion vectors.

SUMMARY OF THE INVENTION

In view of the foregoing, an object of this invention is to provide anapparatus for compressing a motion picture signal and for expanding acompressed motion picture signal in which an original motion picturesignal is compressed with high efficiency to provide a compressed signalthat can be expanded to recover the original motion picture signal.

The foregoing object and other objects of the invention have beenachieved by the provision of an apparatus for compressing a motionpicture signal. The motion picture signal is divided into blocks. Theapparatus comprises a circuit that subtracts the blocks of the motionpicture signal from corresponding prediction blocks of a predictionpicture to provide prediction error blocks; a circuit that orthogonallytransforms the prediction error blocks to provide transformcoefficients; a circuit that quantizes the transform coefficients toprovide quantized transform coefficients; and a circuit that codes thequantized transform coefficients to provide coded transformcoefficients. A local decoding circuit locally decodes the quantizedtransform coefficients to provide an additional prediction picture. Amotion detector calculates a motion vector for each of plural subblocksobtained by dividing each block of the motion picture signal by at leastfour. A representative motion vector generating circuit generates pluralrepresentative motion vectors representing the motion vectors of thesubblocks constituting each block. The representative motion vectors aregenerated from the motion vectors of the subblocks constituting theblock, and are fewer in number than the number of subblocks constitutingthe block. Finally, the apparatus includes a motion compensator forproducing the prediction blocks from the prediction picture by applyingmotion compensation to the prediction picture in response to the pluralrepresentative motion vectors.

The invention also provides a complementary expander for expanding acompressed motion picture signal. The compressed motion picture signalincludes a compressed picture block obtained by compressing a block of amotion picture signal. The compressed picture block includes codedtransform coefficients and coded vector data representing the block ofthe motion picture signal. The coded vector data includes pluralrepresentative motion vectors representing motion vectors of a number ofsubblocks obtained by dividing the block of the motion picture signal byat least four. The expander provides an output picture signal, andcomprises a demultiplexer that separates the coded transformcoefficients and the coded vector data from the compressed pictureblock. A vector decoder detects and decodes the plural representativemotion vectors in the coded vector data. The vector decoder decodesfewer representative motion vectors than the number of subblocks. Acalculating circuit calculates the motion vectors of the subblocks fromthe representative motion vectors. Finally, a circuit derives a block ofthe output picture signal from the coded transform coefficients and themotion vectors.

The nature, principle and utility of the invention will become moreapparent from the following detailed description when read inconjunction with the accompanying drawings in which like parts aredesignated by like reference numerals or characters.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a block diagram illustrating the construction of a fastembodiment of an apparatus for compressing a picture signal according tothe present invention;

FIGS. 2A and 2B are schematic views illustrating a block consisting offlat subblocks and unflat subblocks;

FIGS. 3A and 3B are schematic views for explaining folding in a blockthat includes flat subblocks and unflat subblocks in its lower and upperparts, respectively;

FIG. 4 is a table showing the DCT coefficients generated by the foldingshown in FIGS. 3A and 3B;

FIGS. 5A and 5B are schematic views for explaining folding in a blockthat includes flat subblocks and unflat subblocks in its left and rightparts, respectively;

FIG. 6 is a table showing the DCT coefficients generated by the foldingshown in FIGS. 5A and 5B;

FIGS. 7A and 7B are schematic views for explaining folding in a blockthat includes diagonally-opposed flat subblocks and unflat subblocks;

FIGS. 8A and 8B are schematic views for explaining folding in a blockthat includes only one unflat subblock;

FIG. 9 is a table showing the DCT coefficients generated by the foldingshown in FIGS. 8A and 8B;

FIGS. 10A and 10B are schematic views for explaining a zigzag scan;

FIGS. 11A and 11B are a schematic view of a block and a table of thepixel values thereof, respectively;

FIG. 12 is a block diagram illustrating an apparatus for expanding acompressed picture signal compressed by the apparatus shown in FIG. 1for compressing a picture signal;

FIG. 13 is a block diagram illustrating the construction of a secondembodiment of an apparatus for compressing a picture signal according tothe present invention;

FIG. 14 is a schematic view illustrating a block divided into foursubblocks;

FIG. 15 is a schematic view for explaining the method by which tworepresentative vectors represent the motion vectors of the foursubblocks shown in FIG. 14;

FIG. 16 is a table for explaining how the different patterns shown inFIG. 15 are detected in response to the differential vectors;

FIG. 17 is a table for explaining how the different patterns shown inFIG. 15 are coded;

FIG. 18 is a block diagram illustrating the construction of oneembodiment of an apparatus for expanding a compressed picture signalcompressed by the apparatus shown in FIG. 13 for compressing a picturesignal;

FIG. 19 is a schematic view for explaining the evaluation of arepresentative vector;

FIG. 20 is a block diagram illustrating the construction of thirdembodiment of an apparatus for compressing a picture signal according tothe present invention;

FIG. 21 is a schematic view for explaining the method used in theembodiment shown in FIG. 1 in which the present picture is segmentedinto blocks for carrying out motion compensation;

FIG. 22 is a table for explaining the differential vector, and theadoption block code used in the embodiment shown in FIG. 1;

FIG. 23 is a schematic view for explaining the construction of themotion vector memory 52 used in the embodiment shown in FIG. 1;

FIG. 24 is a table illustrating the variable length coding adopted forthe vector value in the embodiment shown in FIG. 1;

FIG. 25 is a block diagram illustrating the construction of oneembodiment of an apparatus for expanding a compressed picture signalcompressed by the apparatus shown in FIG. 18 for compressing a picturesignal;

FIG. 26 is a schematic view for explaining another embodiment in whichthe present picture is segmented into blocks for carrying out motioncompensation; and

FIG. 27 is a schematic view for explaining a further embodiment in whichthe present picture is segmented into blocks for carrying out motioncompensation.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of this invention will be described with referenceto the accompanying drawings:

FIG. 1 shows a block diagram illustrating the construction of oneembodiment of an apparatus according to the present invention forcompressing a motion picture signal. The principles of the apparatuswill be described first. Discrete cosine transform (DCT) processing isapplied to blocks of the motion picture signal consisting of, e.g., 8×8pixels (8 pixels×8 lines). When a discrete cosine transform is appliedto a block of the motion picture signal, many of the resulting transformcoefficients are zero. Therefore, the number of bits of the compressedpicture signal required to represent the transform coefficients can bereduced by including in the compressed picture signal data indicatingthe number of transform coefficients that are zero. This enables morebits to be allocated for quantizing the non-zero transform coefficients,which, in turn, reduces the quantizing noise. The apparatus shown inFIG. 1 is constructed to reduce further the volume of the compressedpicture signal required to represent the transform coefficients byincreasing the number of zero transform coefficients.

FIGS. 2A and 2B show a block composed of 8×8 pixels segmented into foursubblocks, each consisting of 4×4 pixels. Then, the flatness of eachsubblock is measured. "Flatness," as used herein, indicates that thevariation in the pixel values within the subblock is small.

FIGS. 2A and 2B depict, as an example, an object, the balloon GX, in theright two subblocks, whereas the left two subblocks are devoid of anydetail. More specifically, in this instance, the left two subblocks aredeemed to be flat subblocks, while the right two blocks are deemed to beunflat subblocks. A flat subblock is indicated by a logical 1, whereasan unflat subblock is indicated by a logical 0. The flatness of theblock shown in FIG. 2A may therefore be indicated as shown in FIG. 2B.

The flatness of each subblock is indicated by a logical 0 or alogical 1. Consequently, one block consisting of four subblocks can have16 (=2×2×2×2) possible patterns of flat/unflat subblocks. Hence,flatness information of one block can be indicated by a 4-bit word.

For example, in FIG. 3A, a heart-shaped object is depicted in the uppertwo subblocks, whereas the lower two subblocks are devoid of any detail.In this case, the flatness of the upper two subblocks is indicated by alogical 0, whereas the flatness of the lower two subblocks is indicatedby a logical 1. In this instance, the unflat upper two subblocks arefolded over the flat lower two subblocks about the horizontal centerline L1. This results in the picture shown in FIG. 3B. When a block thatis symmetrical between its upper and lower halves, i.e., about thecenter line L1, is discrete cosine transform processed, alternate rowsof the resulting discrete cosine transform coefficients (transformcoefficients) are all zero, as shown in FIG. 4. Thus, the transformcoefficients in alternate (even-numbered) lines are all zero.

Further, in FIG. 5A, a heart-shaped object is shown in the right twosubblocks in the block, whereas the left two subblocks are devoid of anydetail. In this instance, the unflat subblocks are folded over the flatsubblocks about the vertical center line L2. This results in the pictureshown in FIG. 5B. When DCT processing is applied to the block shown inFIG. 5B, alternate columns of the resulting transform coefficients areall zero, as shown in FIG. 6.

In a further example, FIG. 7A shows a hem-shaped object and astar-shaped object in the left upper subblock and in the right lowersubblock, respectively, of the block, whereas the right upper and leftlower subblocks are devoid of detail. In this case, the pictureillustrated in FIG. 7B results from folding the block about the verticalcenter line L2. Applying DCT processing to this block also resultstransform coefficients, alternate columns of which are zero, as shown inFIG. 6.

In a yet further example, FIG. 8A shows a heart-shaped object in onlythe right lower subblock, whereas the remaining three subblocks aredevoid of detail. In this case, left subblocks are folded over the rightsubblocks along the vertical center line L1, and the upper subblocks arefolded over the lower subblocks along the horizontal line L2. Theresulting block is illustrated in FIG. 8B. When DCT processing isapplied to the block illustrated in FIG. 8B, alternate rows andalternate columns of the resulting transform coefficients are all zero,as illustrated in FIG. 9. In other words, the transform coefficientsshown in FIG. 4 are synthesized with the transform coefficients shown inFIG. 6.

Determining the flatness of the subblocks in a block, and, when two ormore subblocks are flat, performing one or more folding operations toincrease the symmetry of the block, as described above, increases thenumber of zero transform coefficients when the block is DCT processed.This increases the number of quantizing bits available to represent theother, non-zero, transform coefficients in the compressed picturesignal. Instead of including in the compressed picture signal data foreach zero transform coefficient, data are included indicating the numberof zero transform coefficients resulting from the DCT transform of theblock. Since the zero transform coefficients are known from the flatnessinformation, the number of bits available to represent the non-zerotransform coefficients can be increased.

The block of transform coefficients can be read using a zigzag scanalong lines at 45 degrees to the block, as shown in, e.g., FIG. 10A. Theresulting non-zero coefficients and data indicating the number of zerotransform coefficients are included in the compressed picture signal. Inaccordance with the embodiment discussed above, however, rows andcolumns in which all of the transform coefficients are zero appearperiodically, and the locations of these rows and columns are known inadvance. Hence, as illustrated in FIG. 10B, the zigzag-scan can beperformed by skipping the rows and/or columns in which the transformcoefficients are known to be zero. This way, the number of bits requiredto represent the transform coefficients in the compressed picture signalcan be reduced.

In the apparatus shown in FIG. 1 for compressing a motion picturesignal, the input picture signal SIN is supplied to the flat blockjudgment circuit 1, where it is divided into blocks, each block issegmented into subblocks, and the flatness of each subblock is judged.Then, the flat block judgment circuit 1 supplies the folding circuit 2,the switch 3, the variable length coder 6, and the signal multiplexer 7with 4-bit flatness information S1, which indicates which of the foursubblocks constituting the block is flat. Further, the flat blockjudgment circuit 1 calculates a representative value S2 of the flatsubblocks, which it feeds to the signal multiplexer 7.

Each block of the motion picture signal is fed from the flat blockjudgment circuit 1 to the switch 3 either directly, or via the foldingcircuit 2, and thence to the discrete cosine transform (DCT) circuit 4.The transform coefficients from the DCT circuit 4 are supplied to thequantizer 5, where they are quantized. The quantized transformcoefficients from the quantizer 5 are supplied to the variable lengthcoder 6, and the output thereof is supplied to the signal multiplexer 7.In the signal multiplexer, the coded transform coefficients aremultiplexed with the representative value S2 from the flat blockjudgment circuit 1. The multiplexed output is supplied to the outputbuffer 8, where it is temporarily stored. The compressed picture signalSout is read out from the output buffer 8 for recording on a suitablerecording medium (not shown), such as a disc.

Next, the operation of the circuit will be explained. The flat blockjudgment circuit 1 segments the input motion picture signals into blocksof 8×8 pixels (8 pixels×8 lines). Then, each block is segmented intofour subblocks, each having 4×4 pixels (4 pixels×4 lines). Additionally,the flatness of each of the four subblocks is judged. To determine theflatness of a subblock, for instance, if the difference between themaximum and the minimum of the pixel values in the subblock is smallerthan a preset reference value, the subblock is judged to be flat.Alternatively, the flatness can be also judged from, e.g., thedispersion within the subblock. As explained above, the flatnessinformation for each block, indicating the ones of the four subblocksconstituting the block that are flat is a 4-bit code, as stated above.

The flat block judgment circuit 1 also computes the representative valueof each of the subblocks judged to be flat, and feeds eachrepresentative value S2 to the signal multiplexer 7. The representativevalue can be the left upper element A00 in FIG. 11B, which correspondsto the DC component of the transform coefficients. Alternatively, therepresentative value of the subblock can be the mean of the pixel valuesin the subblock. In this case, the 16 pixel values within the subblockare added together, and the resulting sum is divided by 16 (4×4) tocalculate the representative value. One representative value can beprovided for each subblock; or one representative value can be providedfor each block.

The folding circuit 2 folds each block of the motion picture signal fromthe flat block judgment circuit 1, in response to the flatnessinformation supplied by the flat block judging circuit 1. For example,in the block shown in FIG. 11A, if the three subblocks A, B and C areflat, and the subblock D is unflat, the folding process shown in FIGS.8A and 8B is performed. If the pixel values in subblocks A' through D'of the block obtained by folding are indicated by a'ij, b'ij, c'ij,d'ij, respectively, these pixel values are computed by the followingformulae:

    a'ij=d(3-i)(3-j)                                           (1)

    b'ij=d(3-i)j                                               (2)

    c'ij=di(3-j)                                               (3)

    d'ij=dij                                                   (4)

where, as shown in FIG. 11B, dmn represents the pixels of the subblock Dshown in FIG. 11A before the folding process is executed.

The switch 3 switches between its upper and lower contacts as shown inthe figure in accordance with the flatness information supplied from theflat block judgment circuit 1. Consequently, blocks of the motionpicture signal before being folded, or blocks of the folded picturesignal are fed to the discrete cosine transform circuit 4 as required.The discrete cosine transform circuit 4 applies discrete cosinetransform processing to each block of folded or non-folded motionpicture signal. The resulting transform coefficients from the discretecosine transform circuit 4 are supplied to the quantizer 5, where theyare quantized using a predetermined quantizing step size.

The quantized transform coefficients 5 are supplied from the quantizer 5to the variable-length coder 6, where they are variable-length coded.The variable-length coder 6, described above with reference to FIG. 10B,reads each block of transform coefficients by performing a zigzag-scan,and skipping the rows and/or columns in which all the transformcoefficients are zero as a result of the folding. This way, thetransform coefficients are variable-length coded. The flat blockjudgment circuit 1 supplies the variable-length coder 6 with theflatness information S1 to tell the variable-length coder which rowsand/or columns are to be skipped. The variable-length coder 6 determinesfrom the flatness information S1 how the folding was performed, and,hence, the rows and/or column of zero transform coefficients resultingfrom transforming the folded block.

The variable-length coded coefficients from the variable-length coder 6are supplied to the signal multiplexer 7, where they are multiplexedwith the representative value of the subblock S2 supplied by the flatblock judgment circuit 1. The resulting multiplexed signal is suppliedto the output buffer 8, whence the compressed picture signal issubsequently read for recording on the disc (not shown).

FIG. 12 shows the construction of one embodiment of an apparatus forexpanding the compressed picture signal compressed by the apparatusshown in FIG. 1 for compressing a motion picture signal. In theapparatus shown in FIG. 12, the input buffer 11 temporarily stores thecompressed picture signal reproduced from the recording medium (notshown), such as a disc. The demultiplexer 12 separates the compressedpicture signal received from the input buffer 11 into blocks of codedtransform coefficients, representative values, and flatness information.The variable-length coding of the coded coefficients S11 from thedemultiplexer 12 is reversed by the inverse variable length coder 13,and the quantizing of the resulting quantized transform coefficients isreversed by the inverse quantizer 14. The inverse discrete cosinetransform circuit 15 applies an inverse discrete cosine transform toeach block of transform coefficients from the inverse quantizer 14, andthe resulting block of pixel values is fed to the switch 17 directly,and via the restoring circuit 16. The restoring circuit restores thosesubblocks of pixel values that are flat blocks to picture blocks usingthe representative value S2X from by the demultiplexer 12. The switch 17selects blocks of pixel values from the output of the restoring circuit16 or from the output of the inverse discrete cosine transform circuit15 in response to the flatness information S1X.

The operation of the apparatus for expanding the compressed picturesignal will now be described. The demultiplexer 12 separates the codedtransform coefficients from the compressed picture signal read out ofthe input buffer 11, and supplies them to the inverse variable-lengthcoder The demultiplexer 12 also separates the representative value S2Xand the flatness information S1X from the compressed picture signal. Theflatness information S1X is fed to the inverse variable-length coder 13,the restoring circuit 16, and the switch 17, while the representativevalue S2X is fed to the restoring circuit 16.

The inverse variable-length coder 13 applies inverse variable-lengthcoding processing to the coded transform coefficients from thedemultiplexer 12. The inverse variable-length coder, in accordance withthe flatness information S1X from the demultiplexer 12, inserts zeroesinto the rows and/or columns of quantized transform coefficients thatwere skipped in the compressor as a result of folding. The inversequantizer 14 inversely quantizes the quantized transform coefficientsfrom the inverse variable-length coder 13, and feeds the resultingtransform coefficients to the inverse discrete cosine transform circuit15. The inverse discrete cosine transform circuit 15 applies inversediscrete cosine transform processing to each block of transformcoefficients from the inverse quantizer 14, and provides correspondingblocks of pixel values.

As stated above, some of the blocks of pixel values are obtained as aconsequence of folding in the compressor. For these blocks, in responseto the flatness information S1X, the restoring circuit 16 replaces theflat subblock data, which was suppressed by folding in the compressor,with the representative value S2X from the demultiplexer 12. Thesubblocks suppressed by folding are thereby restored. The switch 17selects either the output of the restoring circuit 16, or the output ofthe inverse discrete cosine transform circuit 15. An output signalcorresponding to the original motion picture signal is thereforerestored and fed to the output terminal.

In the system just described, the representative pixel value of the flatsubblocks is included in the compressed picture signal in lieu of thecoded transform coefficients of the flat subblocks. However,transmitting no representative pixel value is also possible. In thiscase, only the flatness information which specifies the flat subblocksis transmitted. In this instance, in the expander, the inverse discretecosine transformation is executed first, to provide the respective pixelvalues. Then, the pixel values in the flat subblocks are obtained bysmoothing accordance with the flatness information. The smoothing methodinvolves, e.g., replacing the respective pixel values or clipping thepixel values in a predetermined range. If no representative pixel valuesare included in the compressed picture signal, it is impossible toreduce the number of transform coefficients by folding.

In the embodiment described above, the blocks of the motion picturesignal are DCT processed. However, the folding process just describedcan be used to reduce the number of bits in the compressed picturesignal can be used with any transform method in which multiple zerocoefficients result from transforming a symmetrical block as in, e.g., aHadamard transform. Further, the present invention is, as a matter ofcourse, applicable to a still picture signal, and is not simply limitedto a motion picture signal.

Next, FIG. 13 is a block diagram illustrating the second embodiment ofan apparatus according to the present invention for compressing a motionpicture signal.

In FIG. 13, the discrete cosine transform circuit 22 applies DCTprocessing to blocks of the motion picture signal SIN, or to a block ofprediction errors between a block of the motion picture signal, and acorresponding prediction block of a prediction picture, generated by thesubtraction circuit 21. The quantizer 23 quantizes the resultingtransform coefficients from the DCT circuit 22. The variable-lengthcoder 24 applies variable-length coding to the resulting quantizedtransform coefficients. The quantized transform coefficients from thequantizer 23 are fed to the inverse quantizer 27, where they areinversely quantized. The inverse discrete cosine transform circuit 28applies inverse DCT processing to the resulting transform coefficientsto provide a block of a reconstructed prediction errors. The block ofreconstructed prediction errors is fed into the adder 34, where it isadded to the prediction block. The resulting reconstructed picture blockis stored in the frame memory 29 as a block of a prediction picture.

The vector arithmetic unit 30 segments the blocks of 8×8 pixels (8pixels×8 lines) of the motion picture signal into four subblocks A, B, Cand D, each having 4×4 pixels (4 pixels×4 lines). The vector arithmeticunit 30 also computes a motion vector VA, VB, VC, and VD for eachsubblock, using the prediction pictures stored in the frame memory 29.This is shown in FIG. 14.

The vector arithmetic unit 30 also computes the following values:

the value VAB of the differential vector between the motion vector VA ofthe subblock A and the motion vector VB of the subblock B,

    VAB=|vector VA-vector VB|                (5)

the value VAC of the differential vector between the motion vector VA ofthe subblock A and the motion vector VC of the subblock C,

    VAC=|vector VA-vector VC|                (6)

the value VAD of the differential vector between the motion vector VA ofthe subblock A and the motion vector VI) of the subblock D,

    VAD=|vector VA-vector VD|                (7)

the value VBC of a differential vector between the motion vector VB ofthe subblock B and the motion vector VC of the subblock C,

    VBC=|vector VB-vector VC|                (8)

the value VBD of a differential vector between the motion vector VB ofthe subblock B and the motion vector VD of the subblock D, and

    VBD=|vector VB-vector VD|                (9)

the value VCD of a differential vector between the motion vector VC ofthe subblock C and the motion vector VD of the subblock D.

    VCD-|vector VC-vector VD|                (10)

The vector arithmetic unit 30 then compares the values of thedifferential vectors with a predetermined threshold THR, and feeds theresults to the block pattern judging unit 31. The block pattern judgingunit 31 selects one of the motion vectors VA, VB, VC, or VD of thesubblocks A, B, C, or D as each of the one or two representative vectorsshown in FIG. 15 for instance.

In FIG. 15, the first pattern PT1 requires a single representativemotion vector to represent the motion vectors VA, VB, VC, and VD of thefour subblocks. The pattern PT1 is the same as if motion compensationwere applied to the complete block.

The second pattern PT2 requires two representative motion vectors torepresent the sets of motion vectors VA and VB, and VC and VD,respectively. The third pattern PT3 requires two representative motionvectors to which represent the sets of motion vectors VA and VC, and VBand VD, respectively.

The fourth pattern PT4 requires two representative motion vectors, oneof which represents motion vector VA and is, for instance the motionvector VA itself, the other of which represents the set of motionvectors VB, VC and VD. The fifth pattern PT5 requires two representativemotion vectors, one of which represents motion vector VB, and is, forinstance, motion vector VB itself, the other of which represents the setof motion vectors VA, VC and VD. The sixth pattern PT6 requires tworepresentative motion vectors, one of which represents motion vector VC,and is, for instance, motion vector VC itself, the other of whichrepresents the set of motion vectors VA, VB and VD. The seventh patternPT7 requires two representative motion vectors, one of which representsmotion vector VD, and is, for instance, motion vector VD itself, theother of which represents the set of motion vectors VA, VB and VC.

The block pattern judging unit 31, in response to the results from thevector arithmetic unit 30, selects one of the available patternsaccording to Table 1 (FIG. 16). The block pattern judging circuit feedsa selected pattern signal, indicating the selected pattern, into themotion compensation circuit 32, which, in response to the selectedpattern signal and the representative motion vectors, performs motioncompensation on the prediction pictures stored in the frame memory 29.

Note that in the Table 1 of FIG. 16, a 0 indicates when the value of thedifferential vector is smaller than the threshold THR, and an Xindicates when the value of the differential vector is larger than thethreshold THR.

The selected pattern signal is also fed into the vector encoder 33.Based on, e.g., Table 2, shown in FIG. 17, the vector encoder generatesthe appropriate variable-length selected pattern code to indicate theselected pattern. The selected pattern code is fed into the multiplexer25, where it is multiplexed with the code from the variable-length coder24.

Next, the operation of the apparatus described above will be described.To reduce redundancy in the time domain, the subtractor 21 derives ablock of prediction errors between a block of the current picture and acorresponding prediction block of a prediction picture read out of theframe memory 29. The block of prediction errors is fed into the discretecosine transform (DCT) circuit 22. The DCT circuit 22 applies a discretecosine transform to the block of prediction errors, and feeds theresulting transform coefficients into the quantizer 23. The quantizerquantizes the transform coefficients, and the resulting quantizedtransform coefficients are then variable-length coded by thevariable-length coder (VLC) 24. The resulting coded transformcoefficients are then fed to the output terminal via the multiplexer 25and the output buffer 26.

In addition, the inverse quantizer 27 and the inverse DCT circuit 28respectively apply inverse quantizing and an inverse discrete cosinetransform to the quantized transform coefficients from the quantizer 23,and the resulting block of reconstructed prediction errors is fed to theadder 34. The block of reconstructed prediction errors supplied to theadder 34 is a reconstruction of the block of prediction errors producedby the subtractor 21.

The motion compensation circuit 32 performs motion compensation on theprediction picture, for example, the previous picture, in response tothe selected pattern signal and the representative motion vectors fromthe block pattern judging unit 31. The prediction block, which is ablock of the prediction picture to which motion compensation has beenapplied according to the selected pattern signal and the representativemotion vectors, is read out from the frame memory 29 and fed to theadder 34 and the subtractor 21. The adder 34 adds the prediction blockto the block of reconstructed prediction errors from the inverse DCTcircuit 28, and the resulting reconstructed picture block is supplied tothe frame memory 29, where it is stored as a block of another predictionpicture.

To generate the prediction block, the motion picture input signal, (e.g.a digital video signal) is fed into the vector arithmetic unit 30, wherethe above-mentioned motion vectors VA, VB, VC, and VD of the foursubblocks A, B, C, and D, respectively, are computed and detected. Thevector arithmetic unit additionally calculates the magnitudes VAB, VAC,VAD, VBC, VBD, and VCD of the differences between pairs of thesevectors, i.e. the differential vectors. The resulting vectors anddifferential vectors are fed into the block pattern judging unit 31,wherein the block pattern for each block is selected. More specifically,the magnitudes of the differential vectors are compared to the thresholdTHR to determine which of the patterns PT1 through PT7 to apply inaccordance with the selection rules shown in Table 1 (FIG. 16).

The resulting selected pattern signal is fed into the vector encoder 33,wherein the selected pattern signal is coded using, for example, thevariable-length codes shown in Table 2 (FIG. 17). The coded patternsignal is fed into the multiplexer 25, where it is multiplexed with thecoded transform coefficients, as described above. In addition, asdescribed above, the selected pattern signal is also fed into the motioncompensator 32, where it employed for providing motion compensation.

Next, FIG. 18 is a block diagram illustrating an embodiment of anapparatus for expanding the compressed motion picture signal compressedby the motion picture signal compressor shown in FIG. 13. The compressedmotion picture signal SINX is fed into the demultiplexer 42 through theinput buffer 41. In the demultiplexer, the compressed motion picturesignal is separated into coded transform coefficients and coded patterninformation. The coded transform coefficients are fed into the inversevariable-length coder (VLC) 43 where the variable-length coding isdecoded. The resulting quantized transform coefficients are inverselyquantized by the inverse quantizer 44, and the resulting transformcoefficients are inverse discrete cosine transformed by the inversediscrete cosine transform circuit 45.

The resulting blocks of prediction errors are, in the same way as in thelocal decoder in the compressor, added to the corresponding predictionblock from the frame memory 49, and stored as a block of a newprediction picture in the frame memory 49.

The coded selected pattern signal is fed into the pattern informationdecoder 47, where it is thereby decoded to provide a selected patternsignal and motion vectors. This information is fed into the motioncompensator 48, where it is used to apply motion compensation to theprediction picture in the frame memory 48. In this manner, thecompressed motion picture signal is expanded to reconstruct the originalmotion picture signal.

Referring now to FIG. 19, a practical example of a method forcalculating a representative motion vector will be described. Therepresentative motion vector is determined by performing block matchingbetween the current block and the prediction picture in each of the twoareas into which the current block is divided. The block matchingresults in the absolute difference sum between the area and theprediction picture being a minimum. The respective symbols are definedas follows:

PO (x, y): pixel value of coordinates in the current picture

PREF (x, y): pixel value of coordinates of the prediction block in theprediction picture

(xA, yA): coordinates of left upper comer of subblock A

(vX, vY): motion vector

L: number of pixels of one side of the subblock

vLIMIT: search range of motion vector

Let sumA (vX, vY) be the absolute difference sum of the subblock A whenthe motion vector is (vX, vY). SumA (vX, vY) is expressed as follows:##EQU1##

The value of (vX, vY) for which sumA (vX, vY) is a minimum in the range-vLIMIT≦vX≦vLIMIT, -vLIMIT≦vY≦vLIMIT is set as the motion vectorVA=(vAX, vAY) of the subblock A.

Similarly, sumB (vX, vY), sumC (vX, vY), sumD (vX, vY) are calculatedand stored, together with sumA (vX, vY). The motion vectors of subblocksB, C and D are also obtained. Then, the differences between thesevectors motion are calculated, from which the block patterns aredetermined using the rules set out in Table 1.

For instance, when the pattern PT2 is selected,

    sumA+B (vX, vY)=sumA (vX, vY)+sumB (vX, vY)                (12)

The value of (vX, vY) for which sumA+B is a minimum in the range of-vLIMIT≦vX≦vLIMIT, -vLIMIT≦vY≦vLIMIT is set as the vector VA+B=(v(A+B)X,v(A+B)Y). The vector VC+D=(v(C+D)X, v(C+D)Y), which gives a maximumvalue of sumC+D (vX, vY)=sumC (vX, vY)+sumD (vX, vY), is calculated in asimilar manner. The resulting vectors VA+B, VC+D are representativemotion vectors.

In the case of the block pattern PT4,

    sumB+C+D (vX, vY)=sumB (vX, vY)+sumC (vX, vY)+sumD (vX,vY) (13)

and the vector VB+C+D is calculated therefrom. The representativevectors are vectors VA and VB+C+D.

In the case of the block pattern PT1,

    sumA+B+C+D (vX, vY)=sumA (vX, vY)+sumB (vX, vY)+sumC (vX, vY)+sumD (vX, vY)(14)

and the representative vector VA+B+C+D is calculated therefrom.

Note that the methods of selecting the block pattern and therepresentative vector are not limited to the embodiment discussed above.For example, another method of selecting the block pattern could involveselecting based on the magnitude of the absolute difference sum withrespect to the prediction picture. Further, as a method of selecting therepresentative motion vector, in the case of, e.g., the block patternPT2, the representative vectors are (vector VA+vector VB)/2 and (vectorVC+vector VD)/2. In the case of the pattern PT4, the representativevectors may be the vector VA and (vector VB+vector VC+vector VD)/3.

Moreover, the block pattern codes are not limited to those set forth inTable 2. Besides, in the embodiment described above, the motion vectorsof the four subblocks are expressed by the two representative motionvectors, but may alternatively be expressed by three representativemotion vectors.

FIG. 20 is a block diagram illustrating a third embodiment of anapparatus for compressing a motion picture signal. In FIG. 20, partscorresponding to those in the embodiment shown in FIG. 13 are designatedby like reference numerals or characters. The motion picture signal,divided into blocks of, for instance, 8×8 pixel values, is fed into themotion vector arithmetic unit 51. The output of the motion vectorarithmetic unit 51 is fed into the motion vector memory 52 and storedtherein. The output of the motion vector arithmetic unit 51 is also fedinto the motion compensator 32, and into the vector encoder 53. Theprediction picture read out data from the frame memory 29 is fed intothe motion compensator 32. The output read from the motion vector memory52 is also supplied to the vector encoder 53. The information receivedby the vector encoder has been processed by the arithmetic equationsdescribed above, so that differential vector information and blockpattern information is encoded therein.

The coded transform coefficients coded by the variable length coder 24are supplied to the multiplexer 25, which multiplexes them with thedifferential vector information and feeds the resulting multiplexedsignal to the output buffer 26. The compressed motion picture signal isread out of the output buffer for recording on a disc (not shown), forinstance.

The operation of the circuit shown in FIG. 20 will be described withprediction to FIG. 21 through FIG. 24. FIG. 21 illustrates how themotion picture signal is segmented into blocks, each of which includes8×8 pixel values, as the units for carrying out motion compensation.Shown in this example is the case in which the motion vectors of threeneighboring blocks A, B, and C, adjacent upwards, right upwards and tothe left of the current block X are to be compared.

Each block has one motion vector for indicating the motion with respectto a prediction block of a prediction picture, one picture before thecurrent block X. More specifically, VX is temporarily defined as themotion vector of the current block X, while VA, VB, and VC are definedas the motion vectors of the neighboring blocks A, B, and C,respectively. Herein, when VX is coded using the fewest bits, processingbased on the following algorithms is executed. Namely;

(1) A first step is to check which of the motion vectors VA or VB or VCequals VX (alternatively, whether or not differences fall within a rangethat is allowable as equal). If two or more vectors are equal to VX,they are represented by one vector; and

(2) Next, if it is judged that any of the vectors VA, VB and VC does notequal VX (alternatively, whether the differences exceed the allowablerange), the vector coded using the shortest length code among the fourvectors, VX-VXA, VX-VXB, VX-VXC, VX is selected (simply, the vectorhaving the least magnitude among the four vectors is selected).

There exist three possible results from the processing of step (1), andfour possible results from the processing in step (2), a total of sevenpossible results from the processing. These results are, as illustratedin FIG. 22, expressed by 3-bit adoption block codes.

In the case of step (1), if it is determined that one of the vectors,VA, VB, and VC is equal to the motion vector VX (alternatively, thedifference falls within the range that is allowable as equal). In thecase of step (2), if it is decided that none of the vectors, VA, VB, andVC is equal the motion vector VX (alternatively, the differences exceedthe allowable range), transmitted also are the differential vectorhaving the shortest code length among four vectors, VX-VXA, VX-VXB,VX-VXC and VX and the adoption block code after being coded.

Note that, if the current block X is located in the outermost row orcolumn of the picture, as can be seen from FIG. 21, the blocks XA and XBof will not exist (when the current block is located in the uppermostrow), or the block XC will not exist (when the current block is in theleftmost column). In such cases, no current block X is disposed in thefirst row and column of the picture, the comparative neighboring blocksdo not exist at all. In such cases, the motion vector VX of the currentblock X is included in the compressed picture signal as it is.

In the motion vector arithmetic unit, the motion vector is computed fromthe current block of the motion picture signal. The motion vector memory52 is capable of storing, as shown in FIG. 23, twice as many motionvectors as the number of blocks in each row of the picture, i.e., themotion vectors for two rows. The motion vector memory 52 holds thevectors of the row that is now being processed, i.e., the current row,and the row before it. The adoption block code and the vector value (ordifferential value) are coded for each block, and the coded result istransferred from the vector encoder 53 to the multiplexer 25.

Further, the variable-length coder (VLC) 24 described above codes thequantized transform coefficients using the VLC codes shown in FIG. 24,which are then fed into the above-mentioned multiplexer 25. The codesfrom the vector encoder 53 are multiplexed therein and transmitted viathe output buffer 26 to a transmission path or a medium such as astorage device or the like (not shown).

FIG. 25 is a block diagram demonstrating one embodiment of an apparatusfor expanding a compressed motion picture signal compressed by thecompressor shown in FIG. 18. In FIG. 25 parts corresponding to ones ofFIG. 18 are designated by same reference numbers or characters. Thecompressed motion picture signal is supplied to the demultiplexer 42 viathe input buffer 41. The demultiplexer separates from the compressedmotion picture signal the coded transform coefficients and thedifferential vector information (i.e., the adoption block code and thedifferential vector code).

The differential vector information separated by the demultiplexer 42 issupplied to the vector decoder 61, to which the motion vector memory 62is connected, to decode the motion vector VX of the current blockaccording to the adoption block code. The output of the vector decoderis supplied to the motion compensator 48, to which the output of theframe memory 49 is connected. The motion compensator performs motioncompensation using the motion vector, and stores the result in the framememory 49 as a prediction block. The prediction block is supplied to theadder 46, where it is added to the block of reconstructed predictionerrors from the inverse discrete cosine transform circuit 45, to providea block of the reconstructed picture. The output of the adder 46 issupplied to the frame memory 49, where it is stored, and is read out asa block of the reconstructed picture signal.

In the above mentioned compressor, the three neighboring blocks, such asXA, XB, and XC, are compared with the current block X. The current blockmay alternatively be compared with two neighboring blocks, such as XAand XB, as shown in FIG. 26, or with four neighboring blocks, such asXA, XB, XC, and XD, as shown in FIG. 27.

In the embodiment illustrated in FIG. 22, the adoption block code neednot be a fixed length code of three bits, but also may be avariable-length code. Although the magnitude of the vectors isvariable-length coded in the manner shown in FIG. 24, the presentinvention is not limited this.

While there has been described in connection with the preferredembodiments of the invention, it will be obvious to those skilled in theart that various changes and modifications may be made therein withoutdeparting from the invention, and it is aimed, therefore, to cover inthe appended claims all such changes and modifications as fall withinthe true spirit and scope of the invention.

What is claimed is:
 1. Apparatus for compressing a motion picture signalto generate a compressed signal, the motion picture signal includingpixel data, and being divided into blocks, the apparatus comprising:flatsubblock judging means for segmenting each of the blocks of the motionpicture signal into subblocks, for determining whether the pixel data ofthe motion picture signal in each of the subblocks have a variation thatis small, and for generating flatness information indicating whethereach of the subblocks is one of a flat subblock and an unflat subblock,the flat subblock being one of the subblocks wherein the pixel data ofthe motion picture signal have a variation that is small; folding means,operating in response to the flatness information generated by the flatsubblock judging means, for modifying the blocks of the motion picturesignal to generate respective blocks of a folded signal, the foldingmeans folding the motion picture signal in a first one of the subblocksin each one of the blocks of the motion picture signal with the motionpicture signal in a second one of the subblocks in the one of the blocksof the motion picture signal, the flatness information indicating thefirst one of the subblocks to be an unflat subblock and the second oneof the subblocks to be a flat subblock; coding means, operating inresponse to the flatness information generated by the flat subblockjudging means, for coding the blocks of the motion picture signal togenerate respective blocks of a coded picture signal, each of the blocksof the motion picture signal being coded in a manner determined by theflatness information; and multiplexing means for multiplexing theflatness information generated by the flat subblock judging means andthe blocks of the coded picture signal generated by the coding means togenerate the compressed signal.
 2. The apparatus of claim 1, wherein:theflat subblock judging means includes representative informationgenerating means for generating representative information representingthe pixel values of the motion picture signal in the subblock indicatedby the flatness information to be a flat subblock; and the multiplexingmeans is additionally for multiplexing the representative information.3. The apparatus of claim 1, wherein the coding means includes:encodingmeans for generating the blocks of the coded picture signal by encodingblocks of an encoding signal obtained by selection between the blocks ofthe motion picture signal and the respective blocks of the folded signalfrom the folding means.
 4. The apparatus of claim 3, wherein theencoding means includes:orthogonal transform means for orthogonallytransforming blocks of the encoding signal to generate respective blocksof a transformed picture signal; and variable coding means for variablycoding the blocks of the transformed picture signal to generate therespective blocks of the coded picture signal.
 5. The apparatus of claim4, wherein:the blocks of the transformed picture signal generated by theorthogonal transform means from the blocks of the encoding signalselected from the folded signal include plural 0 data; and the variablecoding means includes variable-length coding means for performingvariable-length coding by zigzag scanning the blocks of the transformedpicture signal generated by the orthogonal transform means to generatethe respective blocks of the coded picture signal, the variable-lengthcoding means excluding from variable-length coding the 0 data in theblocks of the transformed picture signal generated by the orthogonaltransform means from the blocks of the encoding signal selected from thefolded signal.
 6. The apparatus of claim 5, wherein the variable-lengthcoding means includes means for determining, from the flatnessinformation received from the flat subblock judging means, how thefolding means performed folding in each of the blocks of the foldedsignal.
 7. The apparatus of claim 3, wherein:the flat subblock judgingmeans includes representative information generating means forgenerating representative information representing the motion picturesignal in each of the subblocks indicated by the flatness information tobe a flat subblock; and the multiplexing means is additionally formultiplexing the representative information.
 8. The apparatus of claim7, wherein the encoding means includes:orthogonal transform means fororthogonally transforming the blocks of the encoding signal to generaterespective blocks of a transformed picture signal; and variable codingmeans for variably coding the blocks of the transformed picture signalto generate the respective blocks of the coded picture signal.
 9. Theapparatus of claim 8, wherein:the blocks of the transformed picturesignal generated by the orthogonal transform means from the blocks ofthe encoding signal selected from the folded signal include plural 0data; and the variable coding means includes variable-length codingmeans for performing variable-length coding by zigzag scanning theblocks of the transformed picture signal generated by the orthogonaltransform means to generate the respective blocks of the coded picturesignal, the variable-length coding means excluding from variable-lengthcoding the 0 data in the blocks of the transformed picture signalgenerated by the orthogonal transform means from the blocks of theencoding signal selected from the folded signal.
 10. The apparatus ofclaim 9, wherein the variable-length coding means includes means fordetermining, from the flatness information received from the flatsubblock judging means, how the folding means performed folding in eachof the blocks of the folded signal.
 11. The apparatus of claim 3,wherein the coding means additionally includes selection means,operating in response to the flatness information from the flat subblockjudging means, for selecting between the blocks of the motion picturesignal and the respective blocks of the folded signal from the foldingmeans to provide the blocks of the encoding signal for encoding by theencoding means.
 12. The apparatus of claim 11, wherein:the flat subblockjudging means includes representative information generating means forgenerating representative information representing the pixel values ofthe motion picture signal in each of the subblocks indicated by theflatness information to be a flat subblock; and the multiplexing meansis additionally for multiplexing the representative information.
 13. Theapparatus of claim 12, wherein the encoding means includes:orthogonaltransform means for orthogonally transforming the blocks of the encodingsignal from the selection means to generate respective blocks of atransformed picture signal; and variable coding means for variablycoding the blocks of the transformed picture signal to generate therespective blocks of the coded picture signal.
 14. The apparatus ofclaim 13, wherein:the blocks of the transformed picture signal generatedby the orthogonal transform means from the blocks of the encoding signalselected from the folded signal include plural 0 data; and the variablecoding means includes variable-length coding means for performingvariable-length coding by zigzag scanning the blocks of the transformedpicture signal generated by the orthogonal transform means to generatethe respective blocks of the coded picture signal, the variable-lengthcoding means excluding from variable-length coding the 0 data in theblocks of the transformed picture signal generated by the orthogonaltransform means from the blocks of the encoding signal selected from thefolded signal.
 15. The apparatus of claim 14, wherein thevariable-length coding means includes means for determining, from theflatness information received from the flat subblock judging means, howthe folding means performed folding in each of the blocks of the foldedsignal.
 16. Apparatus for expanding blocks of a compressed signalrespectively derived from blocks of an original motion picture signal togenerate a reconstructed motion picture signal, the original motionpicture signal including pixel data, the blocks of the compressed signalrespectively including blocks of a coded picture signal andcorresponding flatness information, first ones of the blocks of thecoded picture signal including a predetermined number of compresseddata, second ones of the blocks of the coded signal having ones of thecompressed data omitted therefrom, the second ones of the blocks of thecoded signal being respectively derived from blocks of the originalmotion picture signal that include flat subblocks, the apparatuscomprising:inverse multiplexing means for extracting, from the blocks ofthe compressed signal, the blocks of the coded picture signal, and thecorresponding flatness information, the flatness information indicatingwhether each of plural subblocks generated by segmenting the blocks ofthe original motion picture signal wherefrom the blocks of thecompressed signal are respectively derived is one of a flat subblock andan unflat subblock, the flat subblock being a subblock wherein the pixeldata of the original motion picture signal have a variation that issmall; inverse coding means for applying inverse coding to the blocks ofthe coded picture signal extracted by the inverse multiplexing meansfrom the blocks of the compressed signal to generate respective blocksof a transformed picture signal, the inverse coding means includingreplacing means, operating on the blocks of the coded picture signal inresponse to the corresponding flatness information extracted by theinverse multiplexing means, for replacing, prior to the inverse coding,the compressed data omitted from the second ones of the blocks of thecoded picture signal; and inverse transform means for inverselytransforming the blocks of the transformed picture signal in accordancewith a predetermined method to generate respective blocks of thereconstructed motion picture signal.
 17. The apparatus of claim 16,wherein:the inverse coding means additionally includes means forapplying inverse variable-length coding to the blocks of the codedpicture signal extracted from the compressed signal to generaterespective blocks of a first intermediate signal; the replacing meansincludes means for processing the blocks of a first intermediate signalto generate respective blocks of a second intermediate signal, thereplacing means operating to insert into the blocks of the firstintermediate signal derived from the second ones of the blocks of thecoded picture signal one of plural patterns of 0 data corresponding tofolding an unflat subblock with a flat subblock, the one of the pluralpatterns of 0 data for each of the blocks of the first intermediatesignal being indicated by the corresponding flatness information; andthe inverse coding means additionally includes means for inverselyzig-zag scanning the blocks of the second intermediate signal togenerate the respective blocks of the transformed picture signal. 18.The apparatus of claim 16, wherein the inverse multiplexing means isadditionally for extracting, from the compressed signal, representativeinformation representing a pixel value for a subblock of thereconstructed motion picture signal corresponding to each of thesubblocks of the original motion picture signal indicated by theflatness information to be a flat subblock.
 19. The apparatus of claim18, wherein the inverse transform means additionally includes restoringmeans, operating in response to the flatness information and therepresentative information from the inverse multiplexing means, forrestoring to the pixel value represented by the representativeinformation the subblock of the reconstructed motion picture signalcorresponding to each of the subblocks of the original motion picturesignal indicated by the flatness information to be a flat subblock. 20.The apparatus of claim 19, wherein:the inverse coding means additionallyincludes means for applying inverse variable-length coding to the blocksof the coded picture signal extracted from the compressed signal togenerate respective blocks of a first intermediate signal; the replacingmeans includes means for processing the blocks of the first intermediatesignal to generate respective blocks of a second intermediate signal,the replacing means operating to insert into the blocks of the firstintermediate signal derived from the second ones of the blocks of thecoded picture signal one of plural patterns of 0 data corresponding tofolding an unflat subblock with a flat subblock, the one of the pluralpatterns of 0 data for each of the blocks of the first intermediatesignal being indicated by the corresponding flatness information; andthe inverse coding means additionally includes means for inverselyzig-zag scanning the blocks of the second intermediate signal togenerate the respective blocks of the transformed picture signal.